Ordered structures in homogeneous magnetic fluid thin films and method of preparation

ABSTRACT

Methods for preparing homogeneous magnetic fluid compositions capable of forming ordered one dimensional structures or two dimensional lattices in response to externally applied magnetic fields are disclosed. The compositions are prepared using improved co-precipitation methods in which the steps of the procedure have been tuned to reduce sample heterogeneity. Fe 3  O 4  particles are coated with a surfactant and dispersed in a continuous carrier phase to form these homogeneous magnetic fluid compositions. The ability of these compositions to generate ordered structures can be tested by holding a magnet near a thin film of the compositions and observing the formation of colors in the region near the magnet. Methods for controlling the characteristic spacing of the ordered structures formed by the composition also are disclosed. Relevant parameters include the thickness of the film, the strength and orientation of the externally applied magnetic field, the rate of change of field strength, the volume fraction of the magnetic particles dispersed in the continuous phase, and the temperature of the homogeneous magnetic fluid. The homogeneous magnetic fluid composition is useful for the manufacture of liquid crystal devices. The devices take advantage of the serendipitous fact that the spacings in the material are on the order of the wavelength of visible light. A variety of magnetic-optical devices can be constructed that use the ordered structures to diffract, reflect, and polarize light in a controlled and predictable manner. These devices include color displays, monochromatic light switches, and tunable wavelength filters.

RELATED APPLICATION DATA

The present application is related to "Magnetic Fluid Thin FilmDisplays, Monochromatic Light Switch and Tunable Wavelength Filter,"Hong, Chin-Yih Rex; Horng, Herng-Er; Yang, Hong-Chang, and Yeung, WaiBong, Ser. No. 08/835,108, filed concurrently herewith.

FIELD OF THE INVENTION

The present invention comprises methods for producing homogeneousmagnetic fluids capable of forming ordered crystalline structures. Theinvention also comprises methods for generating ordered structures inthin films of such fluids under the influence of externally appliedmagnetic fields, methods for controlling the structures generated inthese films, and magnetic-optical devices based on these orderedstructures. These devices include color displays, monochromatic lightswitches, and tunable wavelength filters.

BACKGROUND

Ferrofluids are a type of magnetic fluid that typically consist ofcolloidal magnetic particles such as magnetite or manganese-zincferrites, dispersed with the aid of surfactants in a continuous carrierphase. The average diameter of the dispersed magnetic particles rangesbetween 5-10 nm. Each particle has a constant magnetic dipole momentproportional to its size that can align with an external magnetic field.

Ferrofluids experience body forces in homogeneous magnetic fields, thatallow their position to be manipulated, and thus enable the constructionof devices such as rotary seals, bearings, and related mechanicaldevices. Ferrofluids also have been used to construct display devicessuch as those disclosed in U.S. Pat. Nos. 3,648,269 and 3,972,595, thatuse a magnetic field to capture an opaque magnetic fluid in apredetermined optical pattern. These types of devices usually operate byhaving an opaque magnetic fluid displace a transparent fluid and therebyproduce optical contrast. Such display devices, however, do not generateordered crystalline structures in the magnetic fluid, and are incapableof generating anything other than a monochromatic image.

Two general methods for producing ferrofluids have been used in theprior art. The first method reduces a magnetic powder to a colloidalparticle size by ball-mill grinding in the presence of a liquid carrierand a grinding aid which also serves as a dispersing agent. Thisapproach is exemplified in U.S. Pat. Nos. 3,215,572 and 3,917,538. Thesecond approach is a chemical precipitation technique as exemplified inU.S. Pat. No. 4,019,994. Both of these techniques suffer from thedisadvantage that there is heterogeneity in the size distribution of theresulting magnetic particles, the composition of these particles, and/orthe interaction forces between the particles. This heterogeneity mayproduce deleterious effects on the ability of a ferrofluid to formordered structures under the influence of a magnetic field.

Pattern forming systems of magnetic fluid films under the influence ofexternal magnetic fields have recently attracted much interest. Forthese studies, a variety of different types of magnetic fluids have beenused. For example, the aggregation process and one-dimensional patternsformed in suspensions of latex or polystyrene particles loaded with ironoxide grains under the influence of parallel fields have been studied byM. Fermigier and A. P. Gast, J. Colloidal Interface Sci. 154, 522(1992), and D. Wirtz and M. Fermigier, Phys. Rev. Lett. 72, 2294 (1994).Quasi two dimensional periodic lattices have been reported to be formedin a phase separated magnetic fluid thin film under the influence of aperpendicular magnetic field. Wang et al., Phys. Rev. Lett. 72, 1929(1994). FIG. 1 of this paper, however, shows that the resultingstructure is disordered. Other investigators have generated more highlyordered two dimensional lattices in thin films of magnetic fluidemulsions or magnetic fluids containing non-magnetic spheres usingperpendicular magnetic fields. However, these lattices tend to solidifyand therefore are not suitable for applications requiring rapidinterconversion between crystalline and amorphous states. See, e.g., Liuet al., Phys. Rev. Lett. 74, 2828 (1995), Skjeltorp, Phys. Rev. Lett.51, 2306 (1983). Thus there is a recognized need in the art forferrofluidic compositions that could be used to generate liquid-crystaldevices that could be switched by small magnetic fields. See, e.g., daSilva and Neto, Phys. Rev. E. 48, 4483 (1993).

If a ferrofluid composition capable of reversibly forming ordered onedimensional structures or crystalline two dimensional lattices in a thinfilm under the influence of an external magnetic field could bemanufactured, it would be useful for constructing a variety of new anduseful liquid-crystal magneto-optical devices. For these reasons, amethod is needed for generating homogeneous ferrofluidic compositionscapable of reversibly forming ordered one dimensional structures orcrystalline two dimensional lattices in a thin film under the influenceof an external magnetic field. Also needed is a simple method fordetermining whether a thin film of a ferrofluidic composition is capableof generating well-ordered one dimensional structures or two dimensionallattices under the influence of external magnetic fields. Finally, itwould be desirable to generate magneto-optical devices based on theordered structures created in thin films of ferrofluidic compositions inresponse to external magnetic fields. Because the utility of suchdevices would be enhanced by developing methods for controlling theordered structures formed in magnetic thin films of ferrofluids underthe influence of external magnetic fields, methods for controlling theordered structures so formed also are needed.

SUMMARY OF THE INVENTION

The present invention is directed to methods for generating homogeneousferrofluidic compositions that are capable of forming ordered structureswhen a thin film of the fluid is subjected to an external magneticfield, as well as compositions synthesized according to this method. Themethod is based on an optimized co-precipitation technique. Theinvention also provides for methods of generating ordered onedimensional structures or two dimensional lattices in thin films ofthese ferrofluidic compositions in response to externally appliedmagnetic fields, as well as methods for determining the ability of ahomogeneous magnetic field to form ordered structures. The inventionalso is directed to the ordered arrays formed in thin films of thehomogeneous ferrofluidic compositions upon exposure to an externalmagnetic field. Also provided are methods for controlling thecharacteristic spacings of the one dimensional structures or twodimensional lattices by varying parameters such as the strength of theapplied magnetic field, the orientation of the field to the film,wherein the angle between the external magnetic field and the plane ofthe film is 0° to 90°, the rate of change of magnetic field strength,the film thickness, the concentration of magnetic particles in theferrofluidic composition, or the temperature of the composition.Further, the invention provides for liquid-crystal magnetic-opticaldevices based on ordered structures created in thin films of ferrofluidsand the ability to control the spacings of these structures. Thesedevices include: a light diffraction color display, a monochromaticlight diffraction switch that can be turned on or off, a tunable lightdiffraction wavelength filter, a second type of light diffraction colordisplay that combines the technologies of the first light diffractioncolor display and the monochromatic light diffraction switch, and alight double refraction color display comprising the magnetic fluid thinfilm of the present invention and polarizers.

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawing(s) will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

FIG. 1 is a flow chart diagram of the steps for preparation of ahomogeneous ferrofluid capable of forming ordered one dimensionalstructures or two dimensional lattices when a thin film of the fluid issubjected to an external magnetic field.

FIG. 2 illustrates a setup for measuring the properties of ferrofluidicthin films under externally applied magnetic fields.

FIG. 3 illustrates two-dimensional hexagonal arrays with particlecolumns occupying lattice vertices generated in a homogeneousferrofluidic thin film in response to an externally applied magneticfield oriented perpendicularly to the plane of the film.

FIG. 4 shows two-dimensional hexagonal arrays formed in films withdifferent thicknesses in response to a perpendicular, 100 Oe magneticfield.

FIG. 5 is a graph showing the relation of the distance between particlecolumns in two-dimensional hexagonal arrays to magnetic field strengthand film thickness.

FIG. 6 is a graph showing the relation of the distance between particlecolumns in two-dimensional hexagonal arrays to the magnetic fieldstrength and the rate of change of magnetic field strength.

FIG. 7 is a graph relating the distance between particle columns intwo-dimensional hexagonal arrays to the magnetic field strength and thevolume fraction ratio between the magnetic particle and liquid carriercomponents of the ferrofluid.

FIG. 8 illustrates the relationship between the periodic spacing ofparticle chains formed in a homogeneous ferrofluidic thin film and thestrength of an external magnetic field that is parallel to the plane ofthe film.

FIG. 9 illustrates the relationship between the periodic spacing ofparticle chains formed in a homogeneous ferrofluidic thin film exposedto a parallel external magnetic field as a function of film thickness.

FIG. 10 illustrates a setup used for demonstrating light diffraction anddouble refraction phenomena generated by ordered structures inhomogeneous ferrofluidic thin films.

FIG. 11 shows a spectrum of colors produced by a magneto-optical devicein which the thickness of the homogeneous ferrofluidic thin film variesfrom about 2 to 10 μm. ##EQU1##

FIG. 12 shows different colors produced by a magneto-optical devicecomprising a homogeneous ferrofluidic thin film as the externallyapplied magnetic field strength is varied.

FIG. 13 illustrates the cross-section of a homogeneous ferrofluidic thinfilm for a first type of light diffraction display device.

FIG. 14 illustrates the design of an individual pixel element comprisinga homogeneous ferrofluidic thin film, a means for generating a magneticfield, and a means for controlling the strength of the field.

FIG. 15 illustrates a cross section of a homogeneous ferrofluidic thinfilm for a double refraction display device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The chemical synthesis of magnetite (Fe₃ O₄) by co-precipitation ofFeSO₄ and FeCl₃ in the presence of NaOH is based on a reaction proposedby W. C. Elmore in 1938. This co-precipitation reaction has been used togenerate ferrofluids (also referred to as "magnetic fluids") in whichthe magnetite particles are coated with a surfactant and dispersed in acontinuous phase (i.e., the particles are dispersed in a liquid that isnot an emulsion). See, e.g., Fertman, V. E., "Magnetic Fluids GuideBook: Properties and Application," Hemisphere Publishing Corporation,1989, ISBN-0-89116-956-3 at page 14. While such materials have provedextremely useful for the construction of various mechanical and displaydevices, they have not been amenable to forming ordered structures inthin films. Ordered structures are regular, periodic arrays of objects,that interact with electromagnetic radiation (e.g., visible light) togenerate physical phenomena such as diffraction or polarization. Thesestructures may be ordered in two dimensions (e.g., x and y), or onedimension (e.g., x). The former structures are sometimes also referredto as lattices, crystalline arrays, or 2-dimensional crystals. Bycarefully tuning the parameters of the co-precipitation reaction andsubsequent coating and dispersing steps, we have synthesized improvedferrofluidic compositions capable of reversibly forming orderedstructures in thin films under the influence of external magneticfields.

While not wishing to be bound by any particular theory, it seems likelythat improvements in the homogeneity of particle size distributionand/or interaction forces between the particles might be responsible forthe ability of these ferrofluidic compositions to form orderedstructures. Improvements in interaction force homogeneity in theferrofluidic compositions of the instant invention may reflect reducedcontamination of the compositions by Fe₂ O₃ and/or water.

According to the methods of the present invention, a composite materialcomprising ultra-fine magnetic particles uniformly dispersed in acontinuous liquid phase is prepared by a co-precipitation techniquewhose controlling parameters were carefully tuned. The magneticparticles are Fe₃ O₄ (magnetite) and result from a chemical reactionbetween a mixture of FeSO₄ and FeCl₃ and alkali such as NaOH, Fe(OH)₂,or Fe(OH)₃. The particles are coated with a layer of surfactant toprevent agglomeration, and are dispersed throughout a continuous liquidcarrier phase to form a homogeneous magnetic fluid. FIG. 1 shows a flowchart diagram of the steps used to prepare a homogeneous magnetic fluidaccording to the present invention.

The general procedure used involves making an aqueous solution of FeSO₄and FeCl₃. The temperature of the solution is maintained at 80° C. andis continuously stirred while a sufficient amount of a hydroxidecontaining base solution such as NaOH, Fe(OH)₂, or Fe(OH)₃ is rapidlyadded to keep the pH of the solution between approximately 11 and 11.5.It is important that not more than about 2 minutes elapse between thestart of the base addition, and the attainment of the target pH value.The co-precipitation of Fe₃ O₄ occurs over about a 20 minute timeperiod. The formula for this reaction is as follows:

    8NaOH+FeSO.sub.4 +2FeCl.sub.3 →Fe.sub.3 O.sub.4 ↓+Na.sub.2 SO.sub.4 +6NaCl+4H.sub.2 O

After about 20 minutes, a surfactant such as oleic acid is added to thesolution out of which the Fe₃ O₄ has precipitated. This serves to coatthe Fe₃ O₄ particles. If the surfactant added is oleic acid, the pHvalue drops substantially at first, and an additional amount of basesolution is added to keep the pH at a preferred range from about 9.5 toabout 10 during the coating process. During the coating process, thetemperature of the reaction is maintained at 80° C. This process takesaround 30 minutes. At the end of this step, the reaction mix separatesinto three phases. Prior to proceeding to the next step, the upper layeris removed and discarded, and the middle and bottom layers are retainedfor use in the next step of the process. The formula of the chemicalreactions that occur during the coating process are as follows whenoleic acid is used as the surfactant and NaOH is used as the base:

    CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COOH+Na.sup.+ OH.sup.- →CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ +H.sub.2 O                                       1.

    Fe.sub.3 O.sub.4 +CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ →Fe.sub.3 O.sub.4. CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ !            2.

After the coating process has completed (around 30 minutes), anacidification step is carried out to protonate the carboxylate group andthereby replace the Na⁺ counterion with a proton. This is achieved byadding a sufficient amount of an acid such as HCl to the reaction mix asit is stirred to bring to the pH of the mixture down to a range of fromaround 0 to around 1. This step is carried out at room temperature (fromabout 20° C. to about 25° C.). The mix is stirred for approximately 20minutes. During this time, magnetic particles coated with surfactantbegin to coagulate. At the end of approximately 20 minutes, the mixphase separates into two layers. The top phase is removed, and theacidification step may be repeated as before an additional two or threetimes. At the end of each acidification step cycle, the top phase isremoved prior to repeating this step. When the top phase no longercontains dark particulate material, the next step may be performed. Theformula of the chemical reactions occurring during this replacement stepis as follows when HCl is used as the acid:

    Fe.sub.3 O.sub.4. CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.- Na.sup.+ !+H.sup.+ Cl.sup.- →Fe.sub.3 O.sub.4. CH.sub.3 (CH.sub.2).sub.7 CH═CH(CH.sub.2).sub.7 COO.sup.31 H.sup.+ !+Na.sup.+ Cl.sup.-

The next step is decantation. During this step, de-ionized water isadded to remove remaining counter ions such as HCl and NaCl from thesurfactant-coated Fe₃ O₄ product. A sufficient amount of de-ionizedwater at 65° C. is added to the coated Fe₃ O₄ to bring the pH of thesuspension to a value between around 4.7 to 5.0. The suspension isstirred as the water is added. After a sufficient amount of de-ionizedwater has been added, the stirring is stopped and the suspension isallowed to settle. The water is decanted away from the settled Fe₃ O₄and the product is washed.

Washing is achieved by adding a liquid used as a carrier (e.g.,kerosene) to the settled Fe₃ O₄ in a ratio of approximately 1.1milliliter of kerosene per gram of coated Fe₃ O₄. The two components arestirred until the solid Fe₃ O₄ is completely suspended in the carrier.This suspension is placed in a centrifuge tube and subjected to a short,low-speed spin carried out at room temperature. We have found that a 10minute spin at a relative centrifugal force equivalent to about 500×gworks well when kerosene is used as the carrier. When the sample isremoved from the centrifuge, it will have separated into two phases. Thetop phase is a dark-colored liquid that contains salt residues and largeparticles, while the lower phase is a solid that contains magneticparticles coated with surfactant. The top phase is removed, and thecoated magnetic particles are dehydrated as completely as ispracticable.

We have found that suitable dehydration can be achieved by suspendingthe particles in acetone, pelleting them with a 30 minute centrifugationat 1800×g, removing the acetone, and drying the particles for 8 to 12hours in a 65° C. oven. After the particles have been dehydrated, theyare dispersed in the carrier, and the fluid is subjected to anothershort, low-speed spin in a centrifuge. This spin pellets larger oraggregated particles. The liquid sitting above any pellet that may beformed in this spin is the homogeneous magnetic fluid of the presentinvention. The concentration of magnetic particles in the fluid may beincreased by setting the fluid in a 65° C. oven for 8 to 12 hours toevaporate a portion of the carrier.

In addition to the kerosene and oleic acid combination described above,other pairs of carriers and surfactants may be used to generate thecompositions of the present invention that are capable of formingordered structures in thin films. Table 1 sets out representativecombinations. In this table, any of the carriers listed in a cell may beused with any of the surfactants listed in the cell in the same row ofthe table.

                  TABLE 1    ______________________________________    Carrier/Surfactant Combinations Useful for Generating Homogeneous    Fe.sub.3 O.sub.4 Magnetic Fluids    Carrier          Surfactant    ______________________________________    1. kerosene      1. oleic acid    2. cyclohexane (C.sub.6 H.sub.12)                     2. linoleic acid    3. n-octane (C.sub.8 H.sub.18)                     3. olive oil, a mixture of:    4. n-dodecane (C.sub.12 H.sub.26)                     ˜9% CH.sub.3 (CH.sub.2).sub.14 COOH,    5. n-tetradecane (C.sub.14 H.sub.30)                     ˜2% CH.sub.3 (CH.sub.2).sub.16 COOH,    6. n-hexadecane (C.sub.16 H.sub.34)                     ˜80% oleic acid,    7. n-octadecane (C.sub.18 H.sub.38)                     ˜10% CH.sub.3 (CH.sub.2).sub.4 CH═CH--CH.sub.2                     --    8. n-eicosane (C.sub.20 H.sub.42)                     CH═CH--(CH.sub.2).sub.7 --COOH                     4.  R(COO)!.sub.2 Zn (where                     R═CH.sub.3 (CH.sub.2).sub.n, and 4 ≦ n                     ≦ 5)                     5. erucic acid    perfluoroeicosane (C.sub.20 F.sub.42)                     1. oleic acid                     2. perfluoropolyether acid                     CF.sub.3 CF.sub.2  CF.sub.2 OCF(CF.sub.3)!.sub.5 COOH    gas oil, C.sub.12 and above hydrocarbon                     1. oleic acid                     2. olive oil                     3.  R(COO)!.sub.2 Zn(where                     R═CH.sub.3 (CH.sub.2).sub.n, and 4 ≦ n                     ≦ 5)    perfluoro kerosene                     1. perfluoropolyether acid                     2. oleic acid                     3. olive oil    2-methoxyethyl ether                     mixture of R--O--R' (where R or                     R'═CH.sub.3 (CH.sub.2).sub.n, and 4 ≦ n                     ≦ 5)                     and R need not equal R'    ______________________________________

Characterization of the homogeneous magnetic fluid

Based on the procedure outlined above, a homogeneous magnetic fluid isprepared. X-ray diffraction patterns of the sample can be used to verifythe single phase fcc spinel structure expected for an Fe₃ O₄ sample. TheX-ray diffraction data can be compared to a standard obtained from theInternational Center for Diffraction Data compiled by the JointCommittee on Powder Diffraction Standards. The magnetization of thesample is measured using a vibration sample magnetometer such as a VSMController Model 4500 available from EG&G Princeton Applied Research.The particle size of the sample is determined from the magnetization,applied field data (M-H data) by fitting it to the Langevin functionL(α)=M/M_(s) =(coth α-1/α), where α=M_(s) VH/kT, M is the magnetizationof the sample at an applied magnetic field strength, H, M_(s) is thesaturated magnetization of the sample, and V is the volume of aparticle. Thus, because temperature, M, M_(s), k and H are known, V canbe solved for and the radius of the particle determined. The Langevinfunction assumes: (1) a uniform particle size; and (2) independentparticle behavior. In a magnetic fluid, the particle size may bedescribed by a normal distribution, and interactions between particlesoccur because the particles generate magnetic dipoles. Thus, the closerthe agreement between the empirical M-H curve and the calculatedLangevin function, the better the assumptions underlying the Langevinfunction are met. If a magnet is held near a homogeneous magnetic fluidthin film manufactured according to the methods of the presentinvention, a light color appears in the film and moves as the magnet ismoved due to optical effects created by ordered structures formed inresponse to the magnetic field. Such colors are not seen in a EMG 909, acommercially available kerosene-based Fe₃ O₄ ferrofluid obtained fromFerrofluidics Corp. (Nashua, N.H.).

Ordered structures in thin films of homogeneous magnetic fluids

A magnetic fluid synthesized according to the methods outlined above maybe sealed into a number of glass cells with various cell thicknesses toform fluidic thin films. The thin films of the present invention havepreferred thicknesses in the range of from about 1 micron to about 20microns, and more preferably from about 2 microns to about 6 microns (1micron=10⁻⁶ m). In one embodiment of the invention, magnetic fieldsparallel to and perpendicular to the plane of the film may be generatedby Helmholtz coils and by a uniform solenoid, respectively.Alternatively, the solenoid may be replaced by Helmholtz coils forapplication of parallel magnetic fields, if desired. The magnetic fieldstrength of the coils and solenoid may be related to the currentsupplied to these devices by using a gauss meter to measure the magneticfield. The resulting magnetic fields are uniform, with deviations offield strength in the region of the film less than 1%. To characterizethe ordered structures produced by applied magnetic fields in thin filmsof the homogeneous magnetic fluids, we photographed the films using aZeiss optical microscope, and the time evolution of the formation ofpatterns in the films was recorded using a personal computer through aCCD video camera.

FIG. 2 illustrates a setup useful for measuring properties ofhomogeneous magnetic fluid thin films under externally applied magneticfields. The power supply used for generating the magnetic fields iscomputer controlled and is programmed such that the image data isobtained automatically. The program controlling the data acquisition iswritten such that the field strength and the rate of change of fieldstrength can be adjusted. If desired, a delay time may be programmedprior to capturing image data after the field strength has been changedto ensure the pattern has reached a quasi-steady state.

When a homogeneous magnetic fluid thin film prepared according to themethods of the present invention is subjected to a perpendicularlyapplied magnetic field (i.e., the field direction is normal to the planeof the film), initial disorder quantum columns form. If the fieldstrength is increased so that it exceeds a critical value, H_(h), anequilibrium two-dimensional hexagonal structure forms with particlecolumns occupying lattice vertices. If the field strength is increasedto another critical value, H₁, the pattern changes from a hexagonalstructure to a labyrinthine pattern. FIG. 3 illustrates this phenomenonin a 6 μm thin film. In the range of field strength between H_(h) andH₁, the distance between the particle columns is almost linearlyproportional to the inverse of the field strength; the distance betweenthe particle columns is on the order of several microns (FIG. 5). Incontrast, commercially available magnetic fluids only generatedisordered quantum columns under the influence of perpendicularlyapplied magnetic fields.

Other parameters affecting the distance between the particle columnsinclude film thickness, L, (FIGS. 4 and 5), the rate of change of thefield strength, dH/dt, (FIG. 6), the magnetic particle concentration inthe fluid (volume fraction ratio), and temperature, T. The distancebetween columns is directly proportional to the magnetic film thickness(FIG. 5). An increase in the rate of change of field strength (dH/dt)tends to decrease the distance between columns for the same final fieldstrength. This may be due to a boundary effect. In a plot of thedistance (d) between columns (on the ordinate) as a function of fieldstrength (H) (abscissa), a curve generated using a larger rate of fieldstrength change will lie below and to the left of a curve generatedusing a smaller rate of field strength change. Volume fraction ratio maybe adjusted by diluting the magnetic fluid with additional carrier.Decreasing the volume fraction tends to increase the distance betweencolumns, when film thickness and rate of field strength change are heldconstant. Thus, a plot of distance as a function of field strength attwo different volume fractions shows that the d-H curve shifts up and tothe right as the volume fraction is reduced. An increase in temperature(T) results in a decrease in the magnetization of the particles, andproduces an increase in the distance between columns as all otherparameters are held constant.

If a thin film of a homogeneous magnetic fluid of the present inventionis subjected to a parallel magnetic field, the magnetic particles in thethin film agglomerate and form chains parallel to the direction of thefield. As the field strength is increased, these chains tend toaggregate and form coarse, long chains because of their interaction. Aone-dimensional quasi-periodic structure has been observed in thin filmsof homogeneous magnetic fluid of the present invention. The chains existin different layers over the thickness of the film. The distance betweenparticle chains is inversely proportional to the field strength (FIG.8), and proportional to film thickness (FIG. 9).

Magnetic-optical devices using homogeneous magnetic fluid thin films

The present invention also relates to optical phenomena created whenmagnetic waves pass through or are reflected by the controllable orderedstructures produced in homogeneous magnetic fluid thin films of thepresent invention upon exposure to externally applied magnetic fields.To demonstrate these phenomena, the setup illustrated in FIG. 10 wasused to construct and test magnetic-optical devices. The area of thethin film used was 1 cm×4 cm. Helmholtz coils and a uniform solenoidwere respectively used to generate parallel and perpendicular magneticfields. The resulting fields were uniform with a measured deviation offield strength in the vicinity of the thin film of less than 1%. A whitelight source was used (Intralux 500-1 240 Watt halogen lamp, VOLPIManufacturing, Inc., U.S.A., lamp operated at approximately 25% maximumpower). The light rays were made near parallel by passing them through atelescope. Two optical lenses were used to make the near-parallel lightparallel. An aperture was placed between the two lenses to control thesize of the light beam. The parallel white light was reflected by amirror located beneath the thin film. The angle of the mirror to thelight beam was adjustable by turning the mirror plane, resulting in achange of the incident angle of the light to the film. Photographicimages of the light through the thin film were taken using a CCD camerathat was connected to a computer for data acquisition. In addition, aconventional film camera was sometimes used to obtain images of the thinfilm.

FIG. 11 is a photograph of a drop of a homogeneous magnetic fluidexposed to an externally applied perpendicular magnetic field. Thethickness of the drop varies because of surface tension effects. Becausethe spacing of the ordered arrays formed in response to the externalmagnetic field vary as a function of film thickness, a spectrum ofcolors is seen when a source of parallel white light is placed below thefilm. The scale bar corresponds to 2 mm.

FIG. 12 is a series of photographic images of a homogeneous magneticfluid thin film that illustrate diffraction of light by the film underthe influence of an externally applied perpendicular magnetic field. Thescale bar on the figure corresponds to 2 mm. In these images, all theparameters were kept constant, except the current to the solenoid usedto generate the magnetic field. The color of the film changes from redto violet as the magnetic field is altered. These images demonstratethat the color of light passing through the thin film can be controlled,and that monochromatic light can be obtained from a thin film with anarea on the order of several square centimeters.

A display device comprising a plurality of pixels, each of whichcomprises a magnetic thin film with an independent electronic circuitfor controlling the magnetic field or temperature experienced by thefilm can therefore be constructed according to the methods of thepresent invention. By properly adjusting the current in each pixel, apolychromatic image may be displayed.

EXAMPLE 1 Preparation of a homogeneous magnetic fluid composition

500 mls. of an 8 molar solution of NaOH was made by adding 160 g of NaOH(95% grade, Nihon Shiyaku Industries, Ltd.) to a sufficient amount ofde-ionized water to bring the final volume to 500 mls. A second solutionwas made by mixing 0.1 moles of FeSO₄.7H₂ O (98% grade, Showa Chemicals,Inc.) and 0.2 moles of FeCl₃.6H₂ O (97% grade, Showa Chemicals, Inc.) ina sufficient volume of de-ionized water to bring the final volume to 600mls. A glass stirring bar was used to continuously stir the secondsolution while a sufficient volume of NaOH was added to raise the pH andmaintain it between 11 and 11.5, as Fe₃ O₄ precipitated out of thesolution. The addition of NaOH was completed in under about 2 minutes.During this step, the temperature was held at 80° C. The precipitationprocess took about 20 minutes.

50 mls. of oleic acid (Showa Chemicals, Inc.) was added to the solutioncontaining the Fe₃ O₄ precipitate to coat the particles with oleic acid.At first, the pH value dropped substantially, and an additional volumeof the NaOH solution was added to keep the pH at around 10 during thecoating process. During this procedure, the temperature was maintainedat 80° C. The coating process took approximately 30 minutes. At the endof this step, the solution separated into three phases. The upper phasewas removed and discarded, and the middle and bottom phases wereretained for use in the following step.

A volume of HCl (37.52%, Polin) was added to the retained solutionsufficient to bring the pH down to about 1. This step was carried out atroom temperature. The solution was stirred for about 20 minutes, asmagnetic particles coated with oleic acid began to coagulate. At the endof the 20 minutes, the solution separated into two phases. The topphase, containing a black particulate suspension, was removed, and theentire acidification procedure was repeated as before. Again the topphase was removed and discarded. The black particulate suspension wasnot observed in the top phase formed following the second acidificationstep. In some syntheses, this step may have to be repeated an additionalone or two times, until the black particulate suspension is no longerobserved in the top phase, removing the top phase between repetitions ofthe process.

After the acidification steps were completed, de-ionized water at 65° C.was added to the retained bottom phase in order to remove remaining HCland NaCl. The water was added as the material is stirred. A sufficientvolume of water was added to raise the pH of the material to around 5.The solid material was allowed to settle and the water was decanted.

The solid material was washed by adding 1.1 ml. of kerosene per gram ofsolid. This was stirred until the solid material was completelydispersed in the kerosene, and the solution was placed in a centrifugetube and centrifuged for 10 minutes at around 500×g. This and all othercentrifugation steps were carried out at room temperature. Aftercentrifugation, the sample had separated into two layers. The top layerwas a dark colored liquid containing salt residues and large particles,and the lower layer was a solid phase which contained magnetic particlescoated with oleic acid. The upper phase was removed, and the magneticparticles were dehydrated by suspending the material in acetone,pelleting it by centrifugation for 30 minutes at around 1800×g, removingthe acetone, and drying the magnetic particles in a 65° C. oven for 8 to12 hours. Finally, the dried particles were dispersed into keroseneusing a kerosene-to-particle ratio of 2 ml/g. This was centrifuged againat around 500×g for 10 minutes. The supernatant was removed from thetest tube and placed in a 65° C. oven for approximately 10 hours todrive off a portion of the kerosene and thereby raise the Fe₃ O₄concentration. The fluid was removed from the oven and was used to formordered structures in thin films. Aliquots of this homogeneous magneticfluid were sealed into glass cells to form magnetic fluid thin films.

An X-ray diffraction pattern for the Fe₃ O₄ sample verified the singlephase fcc spinel structure of the sample. The lattice constant wasmeasured to be 8.40 Å. The magnetization of the sample was measuredusing a vibration sample magnetometer, and a saturation magnetizationvalue of 10.58 emu/g was measured for the homogeneous magnetic fluid.The volume fraction of the homogeneous magnetic fluid was calculated asthe ratio of the saturated magnetization of the magnetic fluid to thatof the dry Fe₃ O₄ powder. This ratio was 18.9%.

EXAMPLE 2 Two-dimensional ordered structures as a function of appliedfield strength

The setup illustrated in FIG. 2 was used to examine pattern formation ina thin film of the homogeneous magnetic fluid thin film synthesized inExample 1 in response to an externally applied magnetic field orientedperpendicularly to the plane of the film. In this example, the strengthof the applied field was varied. FIG. 3 shows images taken of a 6 μmthick magnetic fluid thin film using a CCD video camera that demonstratethe evolution of the two-dimensional ordered structure pattern fromdisorder quantum columns (FIG. 3a), to an ordered hexagonal structure(FIGS. 3b and 3c), and to a disordered labyrinthine pattern (FIG. 3d).These images illustrate that the distance between columns was roughlyinversely proportional to the field strength in the range between twocritical strengths, H_(h) and H₁.

EXAMPLE 3 Two-dimensional ordered structures as a function of filmthickness

In this example, the effect of film thickness on pattern formation wasexamined. The two-dimensional ordered structures in homogeneous magneticfluid thin films with different thicknesses were investigated using thesetup illustrated in FIG. 2. During this experiment, all parametersremain unchanged except the thickness of the film, which was varied from10 μm to 2 μm by using glass sample cells having different cell depths.FIG. 4 provides examples of images of thin films of the homogeneousmagnetic fluid synthesized in Example 1 taken by the CCD camera using aconstant field strength of 100 Oe in which a two-dimensional hexagonalstructure had formed in the films under investigation. These imagesindicate that the distance between columns is roughly proportional tothe thickness of the films over the range of film thickness examined.

The results obtained in Examples 2 and 3 show that a two-dimensionalhexagonal structure forms in homogeneous magnetic fluid thin films thatare subjected to an externally applied perpendicular magnetic field. Thedistance between columns is closely related to the inverse of the fieldstrength and is roughly proportional to film thickness, at least overthe range of thickness shown in Example 3. FIG. 5 plots the distancebetween columns as a function of film thickness and magnetic fieldstrength.

EXAMPLE 4 Two-dimensional ordered structures as a function of the rateof change of field strength

The effect of the rate of change of the magnetic field strength wasinvestigated, using rates of 5 Oe/s, 50 Oe/s, 100 Oe/s, and 400 Oe/s.FIG. 6 shows the relationship between column distance as a function offield strength using different rates of field strength change (dH/dt).The figure shows that as the rate is increased, the curves are displaceddownward and to the left. That is, as the rate of field strength changeincreases, the distance between the columns decreases.

EXAMPLE 5 Two-dimensional ordered structures as a function of volumefraction ratio

Magnetic fluid samples with varying volume fraction ratios between themagnetic particles and the carrier liquid were made according to themethod of Example 1, except in the final dispersing step, the volume ofkerosene added was altered to vary the volume fraction ratio of thefluid. FIG. 7 shows that a decrease in the volume fraction ratioproduces a shift in the distance versus field strength plots toward theupper right. That is, holding all other parameters constant, a decreasein the volume fraction ratio increases the distance between columns.

In the following two examples, the solenoid was replaced by Helmholtzcoils in the setup shown in FIG. 2. As a result, the orientation of themagnetic field applied to the thin film was parallel to the plane of thefilm.

EXAMPLE 6 One-dimensional ordered structures as a function of appliedfield strength

In this example, the homogeneous magnetic fluid thin film was subjectedto an externally applied magnetic field that was parallel to the planeof the film. As the field was applied, the magnetic particles in thefilm agglomerated and formed chains in the plane of the thin filmoriented along the field direction. These particle chains exist indifferent layers over the thickness of the film. When the field strengthwas increased, the chains became periodic and the distance between thechains decreased proportionately. FIG. 8 shows the effect of varying thefield strength from 100 Oe to 400 Oe on the distance between theperiodic particle chains in the homogeneous magnetic fluid thin film.

EXAMPLE 7 One-dimensional ordered structures as a function of filmthickness

In this example, the effect of thin film thickness on theone-dimensional periodic structures formed in response to parallelmagnetic fields was examined. The homogeneous magnetic fluid was sealedinto glass cells with different cell depths, allowing the effect of filmthickness to be investigated. FIG. 9 shows that the distance betweenparticle chains was found to be proportional to the thickness of thethin film in the range of thickness from 10 μm to 2 μm when all otherparameters were held constant.

EXAMPLE 8 First type of light diffraction color display

When an applied perpendicular magnetic field reaches a critical valueH_(h), a two dimensional column array is formed in a homogeneousmagnetic fluid thin film. Diffraction phenomena occur as a parallelwhite light ray passes through the film, and constructive anddestructive interference occurs as the light rays reach the eyes of aviewer. FIG. 13 is a cross section drawing of arrays formed in ahomogeneous magnetic thin film illustrating the light diffractionconcept. In this Figure, d is the distance between columns in atwo-dimensional column array, θ is the angle formed between the incominglight ray and the direction perpendicular to the plane of the film, θ'is the angle formed between the diffracted rays and the directionperpendicular to the plane of the film, and N is the total number ofmagnetic particle columns diffracting the light. After diffraction, theintensity of the light, I, is ##EQU2## where ##EQU3## and λ is thewavelength of light. The condition under which the light intensity, I,becomes maximum is the same as that under which the light becomesbrightest after diffraction through the film. This condition is ##EQU4##where κ is a non-negative integer.

The angle θ can be designed such that sinθ>>sinθ'. For a fixed angle θ,the color observed by the viewer will not change due to the limitedmovement of the viewer when the viewer is far away from the film.Meanwhile, the condition of κ=0 will never occur. The condition of κ=1is the most interesting and important one. Under this condition, d willbe related to λ by

    d sinθ=λ

If this wavelength λ is within the range of visible light, then the samed will also allow only light with a wavelength of λ/κ for κ=2, 3, . . .to pass through the film. Fortunately, light with these wavelengths areoutside the visible spectrum. The reason for this is that the longestwavelength of light visible to the human eye is about 0.7 μm, and so thewavelength of λ/2=0.35 μm. This wavelength is in the ultraviolet regionof the electromagnetic spectrum and therefore is not visible to thehuman eye. Consequently, the viewer will only observe a singlewavelength of light.

Of course, there will be dispersion for the intensity, I. The degree ofthe dispersion δλ must satisfy the condition δλ/λ=1/N. In the case of atwo dimensional column array of a homogeneous magnetic fluid thin film,N is very large and depends on the area of the film. Thus, δλ/λ is verysmall. If the distance between columns d satisfies d=λ/sinθ, a puremonochromatic color will be observed. Fortunately, the distance betweenthe columns in two dimensional column arrays of homogeneous magneticfluid prepared according to the methods of the present invention is onthe order of several micrometers. Therefore, the array is capable ofdiffracting visible light to produce intensity interference.Furthermore, because the distance d can be manipulated by, e.g.,controlling the strength of the externally applied magnetic field, therate of change of the magnetic field strength (dH/dt), the angle betweenthe magnetic field and the film, the thickness of the homogeneousmagnetic fluid thin film, and/or its temperature, the color of the lightobserved by the viewer can be changed at will.

A display constructed according to the methods of the present inventionwill comprise many pixels. Each pixel is made of a homogeneous magneticfluid thin film with an electronic circuit. The electronic circuit isused to drive the change of the column distance in individual pixels,resulting in a change of color of the outgoing light. FIG. 14 is aconceptual drawing of a pixel. As the distance, d, of the pixels in adisplay device are individually adjusted, the display will generate apolychromatic image.

Diffraction phenomena also will occur in a homogeneous magnetic fluidthin film under the influence of an externally applied parallel magneticfield, according to the diffraction principles set out by Bragg. As thefield is applied to the film, the magnetic particles agglomerate andform chains parallel to the plane of the thin film. The distance betweenchains can be controlled by changes in the field strength. When anincident white light beam forms an angle with the plane of the thinfilm, chains will reflect the beam. Since these chains are in differentlayers inside the film, the reflection of light by chains at differentlayers will interfere, resulting in a very sharp color. Here again, theexternally applied magnetic field can be adjusted to obtain the desiredcolors.

EXAMPLE 9 Monochromatic light diffraction switch

As provided in Example 8, the homogeneous magnetic fluid thin film canbe used to create monochromatic light with wavelength λ from whitelight. Under the same conditions as described in Example 8; i.e.,sinθ>>sinθ' the diffraction occurs only when the column distance in thetwo dimensional column array of the homogeneous magnetic fluid thin filmsatisfies the condition of d sinθ=λ. That is, under a particularstrength of external magnetic field, a monochromatic color of light isdiffracted by the film and passes through it to reach the eyes of theviewer. By adjusting the field strength of the externally appliedperpendicular magnetic field, one should be able to close or open thelight switch. If there is a color dye covering the film, the desiredcolor will appear by opening the switch.

EXAMPLE 10 Tunable wavelength filter by light diffraction

This example also uses the concepts developed in Example 8. The columndistance of the hexagonal structure formed in the homogeneous magneticfluid thin film is adjustable and is around several micrometers. Asmentioned in Example 8, one can select any specific electromagnetic wavewith a wavelength on the order of the column spacing by adjusting thisspacing, d. The design of the homogeneous magnetic thin film and itselectronic circuitry are similar to those illustrated in Example 8,except the area of the thin film may be substantially larger.

EXAMPLE 11 Second type of light diffraction color display

The idea of the second type of light diffraction color display is acombination of the technologies used in Example 8 and Example 9. Thisdisplay consists of a large number of pixels. Each pixel includes threemonochromatic light diffraction switches placed adjacent to each other.Each switch is made of a homogeneous magnetic fluid thin film with anaccompanying electronic circuit for controlling the distance, d. Thelight sources for the three switches are red, green, and blue,respectively. The switches are set to allow only the passage of red,green, or blue light, individually.

By properly adjusting the current in the control circuits, one is ableto turn the monochromatic light switch on or off, and hence allow noneor one of these three colors to pass through its switch. Therefore eachpixel of the display will show either black, red, green, blue, or anycombination of these three colors. When the currents of the switchescomprising the pixels are adjusted individually, the display willgenerate a colorful RGB (red, green, blue) picture.

EXAMPLE 12 Light double refraction color display

This example is an application of the use of homogeneous magnetic fluidthin films under an external magnetic field oriented parallel to theplane of the thin film. Under the applied field, the magnetic particlesagglomerate and form chains in the plane of the film. These chains existat different layers over the thickness of the film. The magnetic fluidinside the thin film becomes an anisotropic medium due to thedirectional arrangements of the particle chains.

The light refraction index n.sub.∥, along the direction of the chainswill be different from the light refraction index, n.sub.⊥, along thedirection perpendicular to the chains. Thus, after traveling a distance,s inside the magnetic fluid, in which s is the thickness of the film,the plane of polarization of a light wave with the electric fieldparallel to the direction of the chains will be different from that withthe electric field perpendicular to the direction of the chains.Denoting these field strengths by E.sub.∥ and E.sub.⊥, they are ##EQU5##where ##EQU6## ω is the frequency of the electromagnetic wave, t istime, and c is the speed of light. These electromagnetic waves interferedue to the different values of n.sub.∥ and n.sub.⊥. This example is anapplication of control of the interference of two light waves byadjusting the strength of the externally applied magnetic field,resulting in changes in the difference between n.sub.∥ and n.sub.⊥. FIG.15 illustrates this invention embodied in this e.

In FIG. 15, two polarizers, with their polar axes perpendicular to eachother, cover both sides of a homogeneous magnetic fluid thin film. Theexternally applied magnetic field is chosen such that its fielddirection forms a 45° C. angle with the polar axes of both polarizers.When light impinges on the polarizer, only light parallel to thedirection of the polar axis of the polarizer will be transmitted. Sincethese two polarizers are perpendicular to each other, the light whichpasses through the first polarizer can not pass through the secondpolarizer. However, when there is an anisotropic medium between the twopolarizers, the electric field of the incoming light is rotated. Thussome of the light will be able to pass through the second polarizer.

In this example, in external magnetic field is applied parallel to thefilm, and the magnetic fluid inside the film becomes anisotropic. As aresult of the difference created between n.sub.∥ and n.sub.⊥, the planeof polarization of the light will be rotated as it passes through thefilm. Therefore, some light will be able to pass through the secondpolarizer and reach the eyes of the viewer. Practically, the intensityof light that passes through both polarizers and the homogeneousmagnetic fluid thin film is proportional to ##EQU7## The condition forthe maximum intensity is ##EQU8## in which κ is a non-negative integer.In this case, s, the thickness of the film can not be changed bychanging the external magnetic field. However, the value of (n.sub.∥-n.sub.⊥) can be changed by changing, e.g., the strength of the externalfield. Thus, one is able to obtain light with the desired wavelength by,e.g., adjusting the strength of the external magnetic field. The pixeland the electronic circuit that drives the magnetic field are similar tothose shown in Example 8, with the only difference being that, in thisexample, the magnetic field is parallel to the plane of the homogeneousmagnetic fluid film.

The above examples are provided to illustrate the invention but not tolimit its scope. Other variants of the invention will be readilyapparent to one of ordinary skill in the art and encompassed by theappended claims. All publications, patents, and patent applicationscited herein are hereby incorporated by reference.

What is claimed is:
 1. A magnetic fluid composition, consistingessentially of: Fe₃ O₄ particles coated with a surfactant and dispersedin a continuous phase carrier liquid, wherein the composition forms acrystalline array when exposed to an external magnetic field thatexceeds a critical field strength.
 2. A method of synthesizing amagnetic fluid composition that forms a crystalline array when exposedto an external magnetic field that exceeds a critical field strength,comprising the steps of:(a) precipitating Fe₃ O₄ from an aqueoussolution consisting of FeCl₃ and FeSO₄ solution by adding a sufficientamount of an hydroxide-containing base to raise the pH of the solutionto and maintain it at between 11 and 11.5, wherein the time elapsed fromthe start of the alkali addition and the attainment of a pH value of 11is less than or equal to 2 minutes; (b) coating the Fe₃ O₄ particleswith a surfactant; (c) coagulating the coated Fe₃ O₄ particles by addinga sufficient amount of an acid to reduce the pH of the liquid containingthe coated particles to pH 1 or below; (d) recovering the coated Fe₃ O₄particles and suspending them in a volume of water sufficient to raisethe pH of the suspension to between 4.7 and 5.0; (e) recovering thecoated Fe₃ O₄ particles from the aqueous suspension; (f) washing therecovered coated Fe₃ O₄ particles in a carrier liquid; (g) dehydratingthe coated Fe₃ O₄ particles; and (h) suspending the coated Fe₃ O₄particles in the carrier liquid.
 3. The method of claim 2, wherein thebase is NaOH, the surfactant is oleic acid, and the carrier is kerosene.4. The method of claim 2, wherein the dehydrating step comprises thesteps of:(a) suspending the coated Fe₃ O₄ particles in acetone; (b)recovering the coated Fe₃ O₄ particles from the acetone; and (c) dryingthe recovered particles.
 5. A method of forming a crystalline arrayhaving array elements that comprise magnetic particle columns thatoccupy lattice vertices, the method comprising: exposing to an externalmagnetic field a thin film of a homogeneous magnetic fluid compositionconsisting essentially of magnetic particles, a surfactant, and acontinuous phase liquid carrier, wherein the strength of the magneticfield exceeds a critical value, the field orientation is not parallel tothe plane of the film, and the composition forms a crystalline array. 6.The method of claim 5, wherein the external magnetic field is orientednormal to the plane of the film.
 7. The method of claim 5, wherein theangle between the external magnetic field and the plane of the film isgreater than 0° and less than or equal to 90°.
 8. A crystalline arraymade by the method of claim
 5. 9. The array of claim 8, wherein thearray is capable of diffracting visible light.
 10. A method of alteringthe distance between the elements of the array of claim 8, comprisingchanging a control parameter.
 11. The method of claim 10, wherein thecontrol parameter is the externally applied magnetic field strength. 12.The method of claim 10, wherein the control parameter is the rate ofchange of the externally applied magnetic field.
 13. The method of claim10, wherein the control parameter is the thickness of the film.
 14. Themethod of claim 10, wherein the control parameter is the volume fractionof the magnetic particles in the fluid.
 15. The method of claim 10,wherein the control parameter is the orientation of the externallyapplied magnetic field with respect to the plane of the film.
 16. Themethod of claim 10, wherein the control parameter is the temperature ofthe film.
 17. A product manufactured according to the process of claim2.